Subacromial impingement syndrome (SIS) is a prevalent cause of shoulder dysfunction, affecting a significant portion of the adult population. It is associated with considerable pain, functional limitations, and disability. The evolution of treatment options, including arthroscopic subacromial decompression (ASAD), necessitates an updated evaluation of clinical outcomes and functional improvements. This study aims to assess the effectiveness of ASAD in patients with SIS by evaluating clinical signs and functional outcomes at three, six, and 12 months postsurgery. A prospective interventional study was conducted from August 2022 to November 2023 at Dr. D. Y. Patil Medical College, Hospital & Research Centre, Pune, India. Twenty-five patients diagnosed with SIS were included. Baseline sociodemographic and clinical characteristics were recorded. Patients underwent arthroscopic decompression, and functional outcomes were measured using the Constant score and American Shoulder and Elbow Surgeons (ASES) score at three, six, and 12 months. Data were analyzed using Statistical Product and Service Solutions (SPSS, version 21.0; IBM SPSS Statistics for Windows, Armonk, NY), with a significance level set at p<0.05. The study cohort comprised 64% females and 36% males, with a mean age of 46-55 years. The majority (72%) were engaged in labor work. The mean body mass index (BMI) was 24.89. Rotator cuff status was intact in 56% of patients, while 44% had partial tears. At baseline, 84% reported pain upon lifting the arm, and 92% experienced loss of motion. Postsurgery, the Constant-Murley score improved significantly from 36% poor at baseline to 92% excellent at 12 months. Similarly, the ASES score increased from a baseline mean of 17-84.9 by 12 months (p<0.01). The acromiohumeral distance increased from 9.7 mm before treatment to 10.4 mm after treatment (p=0.009). No infections or neurological deficits were reported. ASAD significantly improves shoulder function and reduces symptoms in patients with SIS. The Constant-Murley and ASES scores demonstrate substantial improvement over a 12-month follow-up period. The procedure is associated with favorable outcomes and minimal complications, supporting its effectiveness as a treatment modality for SIS.